1. Field of the Invention
This invention relates to a power source control apparatus for controlling a current supply to a laser diode in a semiconductor laser. The semiconductor laser oscillates laser by supplying current to a p-n junction.
2. Description of Related Art
FIG. 10 shows an overall circuit of a conventional power source control apparatus for laser diodes.
Referring to FIG. 10, a power source 1 is composed of a constant voltage source like a D.C. power source and so on. A current control switching element 2 is connected to the power source 1 so as to perform a switching operation by an external signal, i.e. an output command signal PL. The power source 1 is connected to a series circuit of the switching element 2 and a reverse-biased diode 3. A reactor 4 is connected to a junction point "a", where the switching element 2 and the diode 3 are connected in series with each other. The other terminal of the reactor 4 is connected at a junction point "b" to an anode side of laser diodes LD1-LDn, which are connected in series. On the other hand, a cathode side of the laser diodes LD1-LDn is connected at a junction point "c" to a junction point between the power source 1 and the diode 3. Power is supplied to the laser diodes LD1-LDn by switching control of the switching element 2 through the reactor 4.
The laser diodes LD1-LDn respectively emit laser beams to a solid state laser medium or rod 5. Such laser beams are excited and oscillated between a reflecting mirror 7 (100% reflectance) and a semi-transparent mirror 6, so that a laser beam 8 goes out from the semi-transparent mirror 6. In order to supply current to the laser diodes LD1-LDn, a current detector 9 detects a current of the reactor 4. Then, an output of the detector 9 is fed back to an error comparator 10 and compared with a command value ITH of a commander 11. The switching element 2 performs switching control according to a comparison result so as to feed a current of an equal value to the command value ITH. The command value signal ITH is turned on and off by an analog gate 12, which is operated by an output drive signal PL. Consequently, the current supplied to the laser diodes LD1-LDn is on-off controlled so that a constant current set as the command value ITH is supplied to the laser diodes LD1-LDn.
FIGS. 11a-11c show waveforms and a time chart explaining an operation of the control apparatus of FIG. 10.
FIG. 11a shows a waveform of the drive signal PL for turning on and off the analog gate 10. FIG. 11b shows a waveform of a laser diode input current I that is a current supplied to the laser diodes LD1-LDn. FIG. 11c is a waveform of a laser diode applied voltage V that is a voltage applied to the laser diodes LD1-LDn.
When the signal PL becomes on at a timing T1, the switching element 2 turns on. Then, the current I increases at a fixed time constant by an inductance of the reactor 4. When the current I of the reactor 4 reaches the command value ITH at a timing T2, the switching element 2 is on-off controlled to keep the current I at such constant value. At this time, the output of the detector 9 is input into the comparator 10 and compared with the command value ITH, so that the current of the same value as the command value ITH is supplied by the switching control. When the signal PL is switched off at a timing T3, the switching element 2 turns off. Then, the current I of the reactor 4 begins decreasing at a fixed time constant. Since the reactor 4 stores energy, which is decided by the inductance and the current value, the current I is not turned off until a timing T4. The inductance of the reactor 4 stores the energy between the timings T1 and T2 when the current I increases. Therefore, in case the inductance is large, it takes a long time. Especially, when a short width pulse is output between the timings T5-T6, the current I cannot be a rectangular wave as shown by the output current waveform Ill between the timings T5-T7.
Namely, the conventional control apparatus cannot supply an input current of rectangular wave to the laser diodes LD1-LDn in response to a short width pulse. Thus, the output current has a slow response speed, and it is impossible to output a correct pulse current waveform.